In 1923, Arthur Compton helped provide some of the first real-world evidence for Einstein's postulation that photons can act like particles. Using X-rays and some graphite he almost single-handily helped demonstrate our modern understanding of wave-particle duality.
What is the Compton effect?
The Compton Effect, or Compton Scattering, is the term given to what happens to the energy of a photon (like an X-ray or gamma-ray) when it interacts with matter (primarily electrons). When this happens its wavelength increases (or energy/frequency decreases) as it scatters off a target electron.
It was first described by Arthur Holly Compton in 1923 and is a fundamental principle in physics. Compton made his discovery after making extensive experiments bouncing x-rays off the electrons of graphite atoms.
Through his work, he discovered that x-rays that scattered off graphite electrons had a lower frequency and longer wavelength compared to their properties prior to hitting them.
The amount that the frequency changes also depends on the angle of scattering, and the amount to which the ray is deflected from its original path.
A great analogy is a game of pool. Imagine a table with just a cue ball and the 8-ball.
If you strike the cue ball at the 8-ball, which was at rest, the cue ball will hit it and deflect (o scatter) at some angle. It will also lose some of its initial momentum and kinetic energy.
This is very similar to what happens when an X-ray photon "hits" and scatters off an electron.
Who was Arthur Holly Compton?
Arthur Holly Compton was an American physicist who won the 1927 Nobel Prize for Physics for his discovery of the effect named in his honor.
He was born on the 10th of September 1892 in Wooster, Ohio. His family was very academic with his father the Dean of the University of Wooster and his brothers both earning Ph.D.'s at the same University.
Arthur had an early interest in Astronomy and even managed to capture a photo of Halley's comet in 1910. He later graduated from Wooster University with a B.Sc. and later completed an M.A. from Princeton in 1914.
He later earned his Ph.D. in physics in 1916 from Princeton. After working as a physics instructor between 1916 and 1917 he was later awarded one of the first two National Research Council fellowships in 1919 that allowed students to study abroad.
While studying at the University of Cambridge, Compton studied the scattering and absorption of gamma rays.
After returning to the United States, Compton was made the Head of the Department of Physics at Washington University in 1920. It was here that his most important work was completed.
What does the Compton effect prove?
Compton's work effectively provided additional supporting evidence for Einstein's resolution of the photoelectric effect. Within this effect, Einstein postulated that light can occur as discrete particles, rather than waves, that could be used to theoretically produce electricity.
This effect is the basic underpinning of how modern solar cells work.
But when it was proposed, in 1905, it really needed some experimental evidence to confirm the theory. Compton's work provided this and showed that photons do indeed exhibit some particle-like behavior.
For his findings, Compton was awarded the 1927 Nobel Prize for Physics. Since then, many thousands of other experiments have since shown that light can exhibit both wave and particle behavior, a property called wave-particle duality.
It has since become one of the cornerstones of quantum mechanics.
Why is the Compton Effect important?
The Compton Effect is important because it helps demonstrate that light cannot be explained purely as a wave phenomenon. This was in opposition to the long-held belief at the time called Thomson scattering.
This classical theory of an electromagnetic wave scattered by charged particles, cannot explain low-intensity shift in wavelength.
"Classically, the light of sufficient intensity for the electric field to accelerate a charged particle to a relativistic speed will cause radiation-pressure recoil and an associated Doppler shift of the scattered light, but the effect would become arbitrarily small at sufficiently low light intensities regardless of wavelength.
The light must behave as if it consists of particles in order to explain the low-intensity Compton scattering. Compton's experiment convinced physicists that light can behave as a stream of particles whose energy is proportional to the frequency." - eng.libretexts.org.
How was the Compton effect discovered?
Compton made his discovery by scattering of x-rays from electrons in a carbon target and finding scattered x-rays with a longer wavelength than those incidents upon the target.
"Compton's original experiment made use of molybdenum K-alpha x-rays, which have a wavelength of 0.0709 nm. These were scattered from a block of carbon and observed at different angles with a Bragg spectrometer.
The spectrometer consists of a rotating framework with a calcite crystal to diffract the x-rays and an ionization chamber for detection of the x-rays. Since the spacing of the crystal planes in calcite is known, the angle of diffraction gives an accurate measure of the wavelength." - wiki.metropolia.fi.
Interesting facts about the Compton effect
1. The Compton Effect provided the first proof of Einstein's postulation that light can behave as a particle as well as a wave.
2. Compton's discovery was also independently observed by the Dutch physical chemist Peter Debye the very same year.
3. Compton was awarded the Nobel Prize for Physics in 1927 for his discoveries.
4. Compton Scattering is an example of inelastic scattering of light by a free charged particle. Here the wavelength of the scattered light is different from that of the incident radiation.
5. Compton scattering is one of three competing processes when photons interact with matter. At lower energies of a few eV or keV photons can be completely absorbed resulting in an electron being ejected from a host atom.
At higher energies of 1.022 MeV or more, the photon can bombard the host atom's nucleus and cause an electron and a positron to be formed (pair production).
6. During WW2, Arthur Compton was a senior member of the Manhattan Project. In this role, he devoted much of his administrative, scientific and inspiration energies into helping create the world's first atomic weapons.
7. Earlier, in 1922, Compton was able to prove the X-rays can be totally internally reflected from glass and silver mirrors. This allowed for the precise values for the index of refraction and electronic populations of substances.
It also allowed for more precise values for the charge of an electron to be determined.